The Mohs tissue surgical technique, which was developed by Frederic E. Mohs of Madison, Wis., is a method of removing skin tumors such as cutaneous malignancies and certain major carcinomas, and evaluating sections (very thin slices) of the tissue under a microscope. To obtain these sections, a tissue sample must be sliced by a device known in the art as a microtome. The microtome is typically located in a refrigerated unit, called a cryostat, which is capable of maintaining an internal temperature of -20 degrees Celsius or below.
Prior to this sectioning or slicing, the tissue must be mounted on a cryostat chuck. A procedure typically used to mount the tissue sample to a tissue chuck includes placing the tissue sample and a small amount of viscous embedding material such as O.C.T. fluid (a tissue mounting fluid such as this is sold under the brand name Tissue Tek II O.C.T. Compound, by Miles Laboratories, Inc.) onto the tissue mounting surface of the chuck, and cooling the O.C.T. and tissue sample until they freeze solid to the chuck. The chuck and attached tissue sample are then placed into a chuck fixture in the cryostat where the tissue is cut into frozen sections having a thickness of only five to seven micrometers. Each section is then placed on a microscope slide and the section is processed and evaluated by techniques that are well known in the art.
One well known method of freezing the O.C.T. and tissue sample involves placing the chuck, which is typically made of a material having high thermal conductivity such as brass or aluminum, within the cryostat and cooling the chuck to -20 degrees Celsius ("C."). A small puddle of O.C.T. is then placed on the mounting surface of the chuck, and the tissue sample is positioned into the puddle. A heat sink (typically a small mass of brass) also at -20 degrees C., may be lowered onto the O.C.T. and tissue sample to flatten the tissue and accelerate the freezing process. Unfortunately, this inexact method requires the technician or surgeon to manipulate the O.C.T. container and tissue sample obscured by the heat sink, within the confines of the cryostat while their hands are exposed to the -20 degree C. temperature that is maintained therein. Such a method is like ironing a wrinkled shirt, blindly, with a concrete block. Additionally, such manipulation can be dangerous due to the close proximity of the knife that is part of the microtome.
To avoid these problems, another method used by many practitioners involves freezing the O.C.T. and tissue sample to the chuck while the chuck is outside of the cryostat. In this method, the O.C.T. and tissue sample are placed on the mounting surface of the chuck, and then the O.C.T. and tissue sample are sprayed with liquid nitrogen or compressed carbon dioxide until frozen. One of the disadvantages of this procedure is that, as the O.C.T. is sprayed with such cold gas, the gas tends to splatter the O.C.T. off the mounting surface of the chuck. Not only may this require additional clean-up of the chuck and surrounding work area, but if too much O.C.T. is splattered from the mounting surface of the chuck, the remaining O.C.T. may be insufficient to properly support the tissue sample during slicing thereof in the cryostat. Additionally, spraying liquid nitrogen directly on the O.C.T. may cause small bubbles of nitrogen to form in the liquid O.C.T. as some of the liquid nitrogen breaks through the surface of the O.C.T. This may cause the tissue sample to be held less firmly on completion of freezing than if no nitrogen had intruded into the O.C.T.
To avoid splattering the O.C.T., the nitrogen can be sprayed onto the O.C.T. and tissue sample in lighter bursts, or at a greater distance, until the outer surface of the O.C.T. forms a shell. Then the nitrogen can be sprayed more directly onto the O.C.T. to complete the freezing process. While this method prevents splattering the O.C.T., it is a method that consumes both time and excessive amounts of liquid nitrogen. Further, between bursts of liquid nitrogen, moisture from the air is able to condense on the O.C.T., creating a frost layer on the outer surface that tends to insulate the O.C.T. from further cooling (since the frost also forms on the tissue sample, the tissue sample is obscured and cannot be viewed directly, and any anomaly which may occur to the tissue sample during the freezing process may go undetected). As a result, this method avoids the problem of O.C.T. splattering at the expense of consuming significantly greater amounts of liquid nitrogen, which in turn increases the likelihood that the technician will run out of liquid nitrogen, thereby interrupting the tissue mounting procedure and requiring the procedure to be started over.
What is needed is a tissue chuck that aids in the freezing of tissue samples while providing complete visualization without the problems inherent in working within the confines of a cryostat, or spraying coolant directly on the tissue sample and O.C.T.